Recently, a wood powder-containing resin molded article obtained by allowing a thermoplastic resin such as polyethylene and polypropylene to contain wood powder of Hinoki (Chamaecyparis obtusa), Sugi (Cryptomeria japonica) or the like has been studied. Since the wood powder-containing resin molded article contains the wood powder, an amount of thermoplastic resin can be reduced. When a predetermined amount or more of wood powder is contained, the wood powder-containing resin molded article can be incinerated as general waste. However, there is a problem that, as the thermoplastic resin has a higher wood powder content ratio, the thermoplastic resin has a lower fluidity in a molten state, and is difficult to directly injection-mold.
To solve the above problem, there has been proposed a technique of performing injection molding by allowing a thermoplastic resin composition such as polyethylene and polypropylene in a molten state to contain wood powder and also impregnating a fluid in a supercritical state into the thermoplastic resin composition under pressure (for example, see Patent Literature 1).
With the technique, a fluidity of the thermoplastic resin composition in a molten state can be improved by impregnating the fluid in a supercritical state. Thus, the injection molding can be performed even when the thermoplastic resin has a high wood powder content ratio.
Meanwhile, when a pressure of the thermoplastic resin into which the fluid in a supercritical state is impregnated is reduced to become equal to or less than a supercritical pressure of the fluid after the thermoplastic resin is injected into a cavity, the fluid is foamed, to thereby form a foamed layer having fine cells inside the obtained wood powder-containing resin molded article. As a result, the obtained wood powder-containing resin molded article has the foamed layer formed in an inner portion, and a non-foamed layer formed on a surface.
Accordingly, there has been proposed to reduce a weight of the obtained wood powder-containing resin molded article by increasing a volume of the cavity to reduce the pressure of the thermoplastic resin composition, and thereby actively foaming the fluid after injecting the thermoplastic resin composition in a molten state into the cavity.